Many commercial and industrial buildings have a standby electrical generator which provides power in the event of a failure of the electricity from the utility company. Should a storm or another event interrupt the electrical power from the normal source, a control circuit detects the outage and starts the standby generator. When the generator has reached full speed and is generating the nominal output voltage, the control circuit activates an automatic transfer switch, such as shown in U.S. Pat. No. 4,157,461. The switch transfers the connection of selected emergency circuits within the building from the normal power supply lines to the output of the standby generator. These emergency circuits typically include minimal lighting throughout the building, elevators, computers, and other equipment which require power during the emergency.
Heretofore, such automatic transfer switches had two positions, one in which the emergency power source, such as the standby generator, was connected to the building circuits and the other position in which the lines from the utility company were connected to the building circuits. The switch typically did not provide a position at which neither power supply was connected to the building circuits. Therefore, if an electrician had to perform main tenance work on the building circuits, an additional shutoff switch had to be provided to disconnect those circuits from the automatic transfer switch so that neither power supply could be connected to the circuit while maintenance was being performed.